Both have since been involved in long disputes with the hire firm, and subsequently their credit card providers, to have the amounts taken off their accounts. Theirs are the latest in a number of cases which critics say point to a serious lack of regulation in the industry. It also shows the leap of faith that customers have to take when handing card details to a car hire firm.

Ian Wallace, an architect from Maidenhead, Berkshire, was the first to contact Guardian Money, shocked that Goldcar had put more than £5,000 on his credit card. Last August he used the website Amigo Autos to hire a Citroen C3 from Goldcar's depot at Malaga airport. He was given a pre-paid voucher and opted to take out the extra super-collision damage waiver insurance when he picked up the car. At the end of his uneventful four-day trip, he returned the car to the airport to catch his late flight home.

As he was parking in Goldcar's exclusive parking area at about midnight, he was approached by what he took to be a member of staff who was moving cars around the car hire area. Seeing the man had the keys to other cars, he handed his over. The attendant checked the car for damage, and Wallace went off to catch his flight.

The next day he received an email and then a call from Goldcar asking to know the whereabouts of the car, and warning him he was liable for charges of €30 a day for an unreturned car. He replied that he had dropped it off as per his rental, and this was confirmed in a phone call. Concerned that Goldcar would try to hit him with extra costs, he contacted his credit card provider, Barclaycard, to warn it not to accept any unauthorised charges.

His worst suspicions were realised. It soon emerged that Goldcar had placed two charges of €3,000 – a total of £5,200 – on his card on 29 August, the day after he had returned the car. How the company came to the conclusion the car was stolen quite so quickly and decided to charge Wallace's card has not been explained.

Jane Neill's story is slightly different, but with a similar outcome. Neill, from Northampton, hired a car from Goldcar's Jerez depot through website auto-europe last October. She was involved in a serious accident that she says was not her fault. The car she was driving was wrecked, and she had to crawl out of the vehicle.

Because the car's repair costs would have exceeded its value, it was a write-off. However, she has had two separate charges of €2,000 (£3,400 in total) put on her MBNA card, in her case, on consecutive days. Goldcar said these were to pay for "repairs".

Both Wallace and Neill have described how it has been almost impossible to get a proper response from Goldcar. They are both growing increasingly frustrated by the credit card process that appears to allow Goldcar to add huge sums on their cards uncontested. Both point out that in each case the car was insured as part of the rental agreement.

Neill says: "I find it unbelievable that MBNA would initially have allowed two consecutive, separate charges of €2,000 to be taken in Spain, without the account flagging up unusual activity. On MBNA's literature it states 'If you are travelling abroad we recommend you contact us so we know you may be spending abroad as accounts are monitored 24/7 for unusual activity'. I find it astonishing that I may have to pay out for car repair services that never have and never will be made. This is what insurance is for, isn't it?"

The pair have independently tried to argue that the credit card firm is jointly liable to get the matter resolved.

In Wallace's case, a spokeswoman for Barclaycard says the matter has finally been settled in his favour this week – almost six months after the rental. "We put a temporary credit on his account when he made us aware of the situation, and as a result he has not paid any interest while we investigated," she says.

"Last year we initiated a chargeback of the £5,200, but this was declined by Goldcar. We then requested a second chargeback which requires the merchant to present detailed evidence as to why it believes the charge to be fair. We have pointed out that the car was insured. In such cases the merchant has 45 days to respond. No response has been received, and therefore the temporary refund we placed on the account has become permanent. We are sorry that this has taken so long to resolve, but we have to abide by the MasterCard rules."

Meanwhile, MBNA says the terms of Neill's car hire agreement obliges it to consider the disputed charges as authorised transactions.

"We appreciate this is a difficult position and we are currently attempting to further investigate, with the car hire company, whether the car was indeed written off. If these transactions are for repairs that are not legitimate, then we will take steps to pursue this. We have ringfenced this money on her account and frozen any interest while we carry this out as quickly as we can," it says.

David Contreras, a spokesman for Goldcar, said both cases are highly unusual and says the company is doing everything possible to resolve them quickly. "Unfortunately they have both been involved in serious incidents that have led to the loss of the cars entrusted to them and whose responsibility and outcome has yet to be determined," he says. "Both of these cases are still being investigated by the Spanish judicial system so the final outcome has still to be determined."

He said Goldcar Rental has more than 25 years' experience and is one of the leading companies in the car hire sector in Spain. "Last year alone Goldcar had a total of over half a million contracts and the vast majority of customers enjoyed a problem-free and enjoyable rental," he says.

In the meantime, Wallace, who says he will never rent a car from Goldcar again, will be asking his credit card provider to lower his credit limit. "If I'd only had a £2,000 limit on my card Goldcar wouldn't have been able to put through this ridiculous amount. To have a bill of £5,200 hanging over you for months on end has been incredibly wearing," he says.

"As I pointed out to Goldcar, a quick look at the airport's CCTV footage would have shown I drove the car into the airport, but this argument hasn't made any difference. I find it very suspicious that this money was added to my account within 24 hours. How would they know it had been stolen in such a short period?"

The great unknown

One of the problems of booking car hire through a web-based service is that you don't know who will actually supply the vehicle. Few car hire websites run fleets of their own, instead they bulk-buy car rentals from the likes of Europcar, Alamo, and sometimes lesser-known firms. The consumer often only finds out the name of the firm when the booking is completed and you get sent a "voucher". This not only makes it hard to avoid brands with which you have had an unhappy experience in the past, you can also end up renting from a company with no UK presence, or one whose depot is not at the airport.

Guardian Money has had many complaints this year from readers who had a poor experience after renting a car. This week's are the first we have had about Goldcar. However, a read of the Review Centre website suggests a "mixed" consumer experience. The website says 119 users have rated the company – with just 5% recommending the firm. Overall it scores 1.2 out of 5, although that is average for the car hire industry.

On the site, someone complains of the same issue as Wallace and Neill. The unnamed writer had a £2,000 hit on their debit card two weeks after the car was returned to Goldcar's Barcelona depot. Goldcar claimed the writer had damaged the car, but, it is alleged, failed to produce photo evidence, or receipts for repairs. The writer says they tried to "talk to them rationally and professionally as there was clearly some sort of mistake". When "they would not listen nor get back to me" they were forced to hire a lawyer to sue the company. "I won the case and got my money back, but it was a nightmare experience," they say.

In general, hirers should always use a credit rather than a debit card.

Try to avoid rental firms, such as Goldcar, that require drivers to pay for a full tank of fuel upfront. For many short-term rentals it is almost impossible to use a whole tank, so holidaymakers end up paying more than the advertised rate.